Biosensors are used to an increasing extent in modern biological analysis technology and in medical diagnostics. A biosensor consists of a biological detection system for a biological substance and what is referred to as a physical transducer. The substance is “detected” via the biological detection system. Said “detection” is converted into an electronic signal by means of the physical transducer. Frequently used biological detection systems are antibodies, enzymes and nucleic acids. In this case the biological detection systems are usually immobilized (fixed) on the transducer in approximately two-dimensional layers. An immobilization (fixing) can be effected in this case by covalent bindings, by affinity interactions and by hydrophilic or hydrophobic interactions. An overview of a structure consisting of approximately two-dimensional biological detection layers is given by I. Willner and E. Katz in Angew. Chem. (“Applied Chemistry”) 112 (2000), 1230 to 1269.
A device and a method of the type cited at the beginning are known from C. Kösslinger et al., Biosensors & Bioelectronics, 7 (1992), pp. 397 to 404. The surface section of the resonator constitutes a detection system for a substance. The piezoelectric resonator acts as a physical transducer. The piezoelectric layer of the known resonator consists of a quartz crystal. Gold electrodes are attached to the quartz crystal. The quartz crystal is excited by electrical actuation of the electrodes to produce bulk acoustic waves in the form of thickness shear mode oscillations. The resonance frequency is about 20 MHz. One of the electrodes forms the surface section used for sorption of the substance of the fluid. The substance is a macromolecular protein which is present in a liquid and which is physically adsorbed at the electrode. As a result of the adsorption of the protein there is a change in the mass and therefore the resonance frequency of the resonator. The following general relationship applies to the change in the resonance frequency (Δf) as a function of the change in the adsorbed amount of the substance per surface unit (Δm) (cf. G. Sauerbrey, Zeitschrift für Physik (“Journal for Physics”, 155 (1959), pp. 206-222):
                    S        =                                            Δ              ⁢                                                          ⁢              f                                      Δ              ⁢                                                          ⁢              m                                =                                    c              ⁢                                                f                  0                                m                                      ∝                          f              0              2                                                          (        1        )            where S is the mass sensitivity of the resonator, f0 is the resonance frequency of the resonator without adsorbed substance, c is a material-specific constant, and m is the mass of the resonator per surface unit. The mass sensitivity is proportional to the square of the resonance frequency of the resonator. At a relatively low resonance frequency f0 of approx. 20 MHz the mass sensitivity of the known device can be estimated at approx. 1 Hz·ng−1·cm2.
A device and a method of the cited type are also known from V. Ferrari et al., Sensors and Actuators, B 68 (2000), pp. 81-87. The device operates as a mass sensor used for detection of a chemical substance. The piezoelectric layer is a plumbum zirconate titanate (PZT) layer. Layer-shaped electrodes (electrode layers) made of a silver-palladium alloy are applied on opposite sides of the PZT layer. The electrodes and the PZT layer form the piezo acoustic resonator. The resonator can be excited to generate a longitudinal oscillation along the layer thickness of the PZT layer by electrical actuation of the electrodes.
The known resonator has a surface section on which a substance can be sorbed. Toward that end the resonator has a chemically sensitive coating forming the surface section. The chemically sensitive coating is a polymer film which is applied to one of the electrodes. The polymer film is for example polystyrole or polymethylacrylate. Various substances, for example hydrocarbons, can be adsorbed on these polymer films. The adsorption causes a change in the mass of the resonator. As a result of this the resonance frequency of the resonator changes. The extent of the change in the resonance frequency is dependent on the adsorbed amount of the substance. The more substance is adsorbed, the greater is the change in the resonance frequency.
The layer thickness of the PZT layer of the resonator is approximately 100 μm. The electrodes are approx. 10 μm thick. The polymer film is applied for example at a thickness of approx. 3 μm. A lateral extension of the resonator is approx. 6 mm. The resonance frequency of the resonator is approximately 7 MHz. The known device having the piezo acoustic resonator is suitable for the detection of a substance of a fluid. The fluid is either a liquid or a gas or gas mixture.
The resonator of the device is mounted on an aluminum oxide substrate. Thick-film technology (TFT) is the method chosen for manufacturing the resonator or, as the case may be, for attaching the resonator to the substrate. Miniaturization of the device is limited due to the resolution that can be achieved using thick-film technology. Increasing miniaturization is desirable, however. For example, a plurality of resonators are combined to form a resonator array. Each of the resonators forms an array element of the resonator array. The smallest possible resonator elements are required in order to be able to arrange the maximum number of array elements on a substrate of a given size.
In addition to the unfavorable size of the resonator from the point of view of miniaturization, the known device is characterized by a relatively low mass sensitivity (cf. Equation 1) due to the relatively low resonance frequency of the resonator. A low concentration of the substance of the fluid or a slight change in the concentration of the substance in the fluid can only be determined with a relatively large error margin.
A so-called “flexural plate wave” (FPW) sensor is known from H. Baltes, Proceedings of the IEEE, Vol. 86, No. 8, August 1998, pages 1660-1678. The sensor is a device for detecting a substance. The device has a piezo acoustic resonator which is mounted on a semiconductor substrate made of silicon. Vapor deposition methods, CMOS (Complementary Metal Oxide Semiconductor) technology and frontside or backside etching of the semiconductor substrate (bulk micromachining) are used in the fabrication of the device. The electrodes and the piezoelectric layer are arranged on the semiconductor substrate in the form of what is called a “cantilever” in such a way that electrical actuation of the electrodes leads to a lateral oscillation of the resonator at a resonance frequency of approx. 140 kHz. The resonator has a chemically sensitive coating made of polyurethane or polysiloxane. These polymers are suitable for the adsorption and therefore for demonstrating the presence of hydrocarbons containing halogens. The fluid is in particular gaseous. If the fluid is channeled past the surface section formed by one of the polymers, the hydrocarbons are adsorbed on the surface section. The mass of the resonator, and therefore also the resonance frequency of the resonator, changes as a function of the concentration of the hydrocarbons. The lateral extension of the resonator is relatively small. It amounts, for example, to 300 μm. However, the resonator is characterized by a relatively low mass sensitivity due to the relatively low resonance frequency of approx. 140 kHz.